Carbon dioxide is used by plants in photosynthesis (6CO2 + 6H20 -> C6H12O6 + 6O2). Three things happen:
1.Incomplete decay occurs i.e fossil fuels are fromed. When they are burned/combusted they realese carbon dioxide into the athmoshpere.
2.The plant release carbon dioxide due to respiration (C6H12O6 + 6O2 -> 6CO2 + 6H2O). Also other organisms do this.
3.The plant is eaten by animals which also respire but when the animal dies the bones,teeth, and shells release CO2 due to weathering.
And the cycle reapets.
Hope this helps :).